Real‐time volumetric flow measurements with complex‐difference MRI

Blood flow in large vessels can be noninvasively evaluated with phase‐contrast (PC) MRI by encoding the spin velocity to the image phase. Conventional phase‐difference processing of the flow‐encoded image data yields velocity images. Complex‐difference processing is an alternative to phase‐difference methods, and has the advantage of eliminating signal from stationary spins. In this study, two acquisitions with differential flow encoding are subtracted to yield a single projection that contains signal from only those spins moving in the direction of the flow‐encoding gradients. The increase in acquisition efficiency allows real‐time flow imaging with a temporal window as short as two acquisition lengths (60 ms). Validation of the complex‐difference method by comparison with conventional gated‐segmented PC‐MRI in a flow phantom yielded a correlation of r > 0.99. Peak arterial flow rates in the popliteal artery and desending aorta measured in vivo with the complex‐difference method were 0.92 ± 0.06 of the values measured with conventional PC imaging. Real‐time in vivo volumetric flow imaging of transient flow events is also presented. Magn Reson Med 50:1248–1255, 2003. Published 2003 Wiley‐Liss, Inc.